1. Field of the Invention
The present invention relates to a double row bearing, and more particularly to a dual row bearing used for a spindle motor for a precision instrument.
2. Description of the Related Art
As shown in FIG. 1, a conventional dual row bearing as one of the double row bearings includes a shaft 101, a pair of inner rings 102 and 103 disposed in an axially separated manner from each other on the shaft 101, a pair of outer rings 104 and 105 coaxially surrounding the inner rings 102 and 103, two rows of balls 110 and 111 arranged circumferentially on the raceway surfaces 106 and 107 formed in the outer surfaces of the inner rings 102 and 103 and the raceway surfaces 108 and 109 formed in the inner surfaces of the outer rings 104 and 105. A circular spacer 112 is provided between the outer rings 104 and 105 in the conventional dual row bearing. Annular grooves 113 are formed in those portions of the shaft 101 which correspond to the inner surfaces of the inner rings 102 and 103. After the annular grooves 113 have been filled with adhesive, a predetermined preload is applied from one of the inner rings 102 and 103 (in FIG. 1, the left side inner ring 102) towards the other of the inner rings 102 and 103 (in FIG. 1, the right side inner ring 103) and the inner rings 102 and 103 are bonded to the shaft 101.
When the inner rings 102 and 103, the outer rings 104 and 105 and the balls 110 and 111 are made of bearing steel, the outer rings 104 and 105 expand more largely than the inner rings 102 and 103 as the temperature of the dual row bearing mounted on a component of a machine or an instrument is raised, since the inner diameter of the outer rings 104 and 105 is larger than the outer diameter of the inner rings 102 and 103. Thus, the radial space between the outer surfaces of the inner rings 102 and 103 and the inner surfaces of the outer rings 104 and 105 increases due to the temperature rise of the bearing.
Since the balls 110 and 111 between the inner rings 102 and 103 and the outer rings 104 and 105 have much smaller diameters than the outer diameter of the inner rings 102 and 103 and the inner diameters of the outer rings 104 and 105, the increase of the diameter of the balls 110 and 111 is less than the increased distance between the outer surfaces of the inner rings 102 and 103 and the inner surfaces of the outer rings 104 and 105 caused by elevation of the temperature.
This reduces the pushing forces between the balls 110 and the raceway surface 106 of the inner ring 102 and the raceway surface 108 of the outer ring 104 and between the balls 111 and the raceway surface 107 of the inner ring 103 and the raceway surface 109 of the outer ring 105, or, in some cases, the balls 110 and 111 are separated from the raceway surfaces 106 and 107 and/or the raceway surfaces 108 and 109.
When the pushing forces are lowered, the preload of the bearing is remarkably reduced. Or, when the balls 110 and 111 are separated from the raceway surfaces 106 and/or 108 and/or 107 and/or 109, the preload is diminished and the bearing cannot function well.
Since the bearing of this type is very small, its heat capacity is very small. Thus, when this bearing is used in a spindle motor, the bearing is much influenced by heat generated due to the electric consumption power of the coil of the spindle motor, heat generated in the bearing by friction or the like and/or by heat from an external heat source.
The raceway surfaces 106 and 107 of the inner rings 102 and 103 are finished extremely accurately. However, when bonded to the shaft 101, the inner rings 102 and 103 are deformed and sometimes their dimensional accuracy is deteriorated. For instance, the roundness of the raceway surface of the inner rings 102 and 103 sometimes changes from 0.1 xcexcm at the finished state to 0.5 xcexcm to 1 xcexcm or more in a bonded state to the shaft 101. Further, when the inner rings 102 and 103 are bonded to the shaft 101 in an inclined state, the inclination cannot be corrected. This deteriorates the assembly accuracy of the bearing.
Further, this bearing is of a radial type. Thus, when a load which causes an axial relative movement between the shaft 101 and the outer rings 104 and 105 and particularly when such axial relative movements are repeated, it is not desirable to use such bearing.
An object of the present invention is to provide a double row bearing in which a preload does not change quickly even if the temperature of the bearing changes whereby the bearing keeps a high performance characteristic.
Another object of the present invention is to provide a double row bearings having high assembly accuracy.
A further object of the present invention is to provide a double row bearing which has a long service life and which can be used stably even if it is in a condition in which an axial load is applied to the bearing.
In order to achieve the objects of the present invention, there is provided a double row bearing which has a shaft, outer ring means coaxially surrounding the shaft and relatively rotatable therearound. The outer ring means has one end portion and the other end portion.
A first annular raceway surface is formed in the inner peripheral surface in said one end portion of the outer ring means, and an annular inner ring surrounds the shaft between the shaft and said one portion of the outer ring means. A second annular raceway surface is formed in the outer peripheral surface of the inner ring so as to radially face the raceway surface. A row of balls are received by the first and second raceway surfaces and arranged circumferentially thereof.
A preloading member is mounted on the shaft at the end of said one end portion of the outer ring means, and elastic means is provided between the inner ring and the preloading member. The preloading member is fixed to the shaft in a position in which the preloading member presses the elastic means towards the inner ring to apply a predetermined preload to the inner ring through the elastic means.
The elastic means can comprise at least one belleville spring.
Alternatively, the elastic means can comprise at least one gear-shaped spring having tooth sections inclined towards the inner ring.
Alternatively, the elastic means is made of elastic rubber.
A space can be formed between the end of said one end portion of the outer ring means and said one end of the inner ring so that at least a part of the preloading member enters the space. The preloading member can be bonded to said shaft.
An annular groove filled with adhesive is formed in the outer peripheral surface of the shaft. Axial slots filled with adhesive can be formed in the area of the outer peripheral surface of the shaft at an end portion of the shaft and arranged so as to be separated circumferentially of the shaft.
It is preferred that the shaft comprises a first cylindrical shaft portion inserted into the inner ring and a second cylindrical shaft portion formed integrally and coaxially with the first shaft portion and having an outer diameter substantially equal to the diameter of the inner peripheral surface of the other end portion of the outer ring means, wherein a third annular raceway surface is formed in the inner peripheral surface of the other portion of the outer peripheral surface of the outer ring means, a fourth annular raceway surface is formed in the outer peripheral surface of the second shaft portion so as to radially face the third annular raceway surface, and another row of balls are disposed between the other portion of the outer ring means and the second shaft portion so as to be received by the third and fourth annular raceway surfaces arranged circumferentially thereof.
Preferably, a first outer ring is formed by said one end portion of the outer ring means and a second outer ring is formed by the other end portion of said outer ring. In this case, a sleeve-shaped spacer surrounds the shaft and is disposed between the first and second outer rings in an abutment state thereagainst.
Another elastic means can be provided between one end of the second shaft portion and the other end of said inner ring, for applying another urging force to said inner ring, wherein the preloading member is fixed to the first shaft portion in a position in which the predetermined preload is substantially equal to the urging force applied by the first-mentioned elastic means to the inner ring minus said another urging force applied by said another elastic means to the inner ring.
Alternatively, said one end portion of the outer ring means can comprise a first outer ring, and the other end portion of the outer ring means can comprise a second outer ring wherein a sleeve-shaped spacer surrounds the shaft and is disposed between the first and second outer rings so as to abut thereagainst.
The outer ring means can comprise a single integral sleeve-shaped member.
The inner ring can be made narrower than the first outer ring.
It is preferred that one end portion of the outer ring means comprises a first outer ring which the row of balls engage and that the other portion of the outer ring means than said one end portion thereof comprises a second outer ring coaxially surrounding the shaft and abutting against the first outer ring.
It is also preferred that the double row bearing can be constructed such that a first inner race is formed by the inner race, the outer ring means comprises a first outer ring formed by the end portion of the outer ring means and coaxially surrounding the first inner ring and a second outer ring formed by the other end portion of the outer ring means. In this case, an annular spacer surrounds the shaft and is disposed between the first and second outer rings in an abutment state thereagainst. A second inner ring is mounted on the shaft and provided between the shaft and the second outer race. An annular raceway surface is formed in the inner peripheral surface of the second outer ring and another annular raceway surface is formed in the outer peripheral surface of the second inner ring so as to radially face the annular raceway surface of the second outer ring. Further, a row of rollable balls are disposed between the second outer ring and the second inner ring and received by the raceway surfaces of the second outer ring and the second inner ring so as to rotate circumferentially thereof.
Preferably, the double row bearing is used for a spindle motor.